Heart failure (HF) is the leading cause of hospitalization in the adult population of the United States and developed countries, and increases the risk for arrhythmias and sudden cardiac death. In HF, arrhythmias are typically secondary to disrupted regulation of intracellular calcium ([Ca2+]i) or electrical remodeling in cardiac tissue that involves altered expression or function of ion channels and ion channel regulatory proteins. In particular, the slowly activating delayed rectifier K+ current (IKs) is downregulated in HF, and evidence suggests [Ca2+]i can regulate IKs. IKs is a key component of the repolarization reserve of the cardiac action potential (AP), most prominently during ?-adrenergic stimulation. During the cardiac AP, depolarization causes an increase in [Ca2+]i due to Ca2+ entry via L-type Ca channels (ICa) and subsequent sarcoplasmic reticulum (SR) Ca2+ release. The rise in [Ca2+]i at the membrane (submembrane [Ca2+]i) that is sensed by ion channels during each cardiac cycle is larger than cytosolic [Ca2+]i. Thus, the increase of [Ca2+]i during a single Ca2+ transient may dynamically increase IKs, and therefore limit the AP duration and consequently the amount of Ca2+ entry into a myocyte. Furthermore, Ca2+ sensitivity of IKs could reduce the repolarization reserve during conditions where [Ca2+]i dynamics are compromised, such as HF. Although the Ca2+ dependence of IKs is seemingly important, the modulation of IKs by the dynamically changing [Ca2+]i transient has not been studied as thoroughly as ICa, Na-Ca exchanger, or SR Ca2+ release. Therefore, the overall hypothesis of this proposal is that Ca2+ regulation of IKs is dynamic and a key determinant for the physiological contribution of IKs to cardiac repolarization. To examine this hypothesis, Aim 1 will determine the effects of steady-state and dynamic changes of [Ca2+]I on the amplitude and gating properties of IKs. The regulation of IKs during each cardiac cycle by [Ca2+]i, specifically submembrane [Ca2+]i, will reveal the prominent role of IKs within normal cardiac repolarization. This will be evaluated in rabbit ventricular myocytes using the patch-clamp technique with wide-field epifluorescence in an environment consisting of a range of controlled [Ca2+]i (steady-state) and during an AP waveform that allows for triggered SR Ca2+-release (dynamic). Aim 2 will expand upon Aim 1 by measuring the steady-state and dynamic [Ca2+]i regulation of IKs during ?-adrenergic stimulation to determine if this regulation increases the sensitivity of IKs to [Ca2+]i. Lastly, Aim 3 will examine the hypothesis that decreases in IKs in a rabbit model of HF reflect both a decreased sensitivity of IKs to [Ca2+]i and a decreased responsiveness to ?-adrenergic stimulation by using a similar approach as Aims 1 and 2.